In my opinion the reason "portfolio theory" works in the cases that you mentioned is that it most perform well across a time series within a highly dynamic envi

In my opinion the reason "portfolio theory" works in the cases that you mentioned is that it most perform well across a time series within a highly dynamic environment. But if one is allow to pick an optimal team for each discrete problem then it will surely outperform. In my mind diversity will help when you have little to no a priori knowledge and then for can not predict what attributes you will need.